Subclinical hypothyroidism in pregnant rats impaired learning and memory of their offspring by promoting the p75NTR signal pathway

Abstract

Objective: Maternal hypothyroidism during pregnancy can affect the neurodevelopment of their offspring. This study aimed to investigate the effects of maternal subclinical hypothyroidism (SCH) on spatial learning and memory, and its relationship with the apoptotic factors in cerebral cortex of the offspring.

Methods: Female adult Wistar rats were randomly divided into three groups (n = 15 per group): control (CON) group, SCH group and overt hypothyroidism (OH) group. Spatial learning and memory in the offspring were evaluated by long-term potentiation (LTP) and Morris water-maze (MWM) test. The protein expression of the p75 neurotrophin receptor (p75^NTR), phospho-c-Jun N-terminal kinase (p-JNK), the pro-apoptotic protein p53 and Bax were detected by Western blotting.

Results: The Pups in the SCH and OH groups showed longer escape latencies in the MWM and decreased field-excitatory post synaptic potentials in LTP tests compared with those in the CON group. p75^NTR, p-JNK, p53 and Bax expression levels in the cerebral cortex increased in pups in the SCH and OH groups compared with those in the CON group.

Conclusions: Maternal SCH during pregnancy may impair spatial learning and memory in the offspring and may be associated with the increased apoptosis in the cerebral cortex.

Keywords: cerebral cortex; long-term potentiation; p75 neurotrophin receptor signaling pathway; subclinical hypothyroidism.

Figure 1

Schematic of experimental timeline. E0, gestational day 0; LTP, long-term potentiation; MWM, Morris Water Maze; PND, postnatal day.

Figure 2

Thyroid-stimulating hormone (TSH) and serum total thyroxine (TT4) levels in pregnant rats (9 days of injection, n = 10 per group; PND1, n = 6 per group). *P < 0.05 vs same day CON group; ^#P < 0.05 vs same day SCH group. CON, control; OH, overt hypothyroidism; PND 1, postnatal day 1; SCH, subclinical hypothyroidism; TSH, thyrotropin; TT4, total thyroxine.

Figure 3

Performance of pups in the MWM test. Data are expressed as the mean ± s.e.m. (n = 10 for each group). Average time to find the hidden platform was longer in the SCH and OH groups compared with the CON group from PND 40–43.

Figure 4

Probe trial test recorded the time spending by each pup in the platform area. Pups in the SCH and OH groups spent less times than pups in the CON group. *P < 0.05 vs same day CON group; ^#P < 0.05 vs same day OH group.

Figure 5

Probe trial test recorded the number of times pups crossed the platform quadrant. Pups in the SCH and OH groups crossed fewer times than pups in the CON group. *P < 0.05 vs same day CON group; ^#P < 0.05 vs same day OH group.

Figure 6

Maternal SCH caused LTP damage in the hippocampal CA1 region in their pups. LTP was induced by HFS and measured as an increase in f-EPSP slope, expressed as percent of the baseline of f-EPSP slope after HFS in different groups. f-EPSP slopes were reduced in the SCH and OH groups compared with the CON group (P < 0.05).

Figure 7

Protein expression levels of p75 neurotrophin receptor (p75^NTR), phospho-c-Jun N-terminal kinase (p-JNK) and pro-apoptotic proteins p53 and Bax in the cortex of pups (n = 6 per group). *P < 0.05 vs CON group, ^#P < 0.05 vs OH group.

Figure 8

Effects of SCH and OH on p75^NTR signaling pathway. ProNGF, as a precursor protein of NGF, can combine with p75^NTR preferentially. p75^NTR activates the JNK pathway to increase the p53 signaling pathway. p53 and Bax play essential roles in JNK-mediated neuronal apoptosis. In the present study, maternal SCH and OH increased p75^NTR expression in the offspring, thus increasing activation of p-JNK, Bax and p53 signaling pathways and thereby increasing neuronal apoptosis.